I discovered that myself some days ago. This is due to that later versions of OpenSSL (at least 0.9.8o, 1.0.0.a) has added an include of <winsock.h> in include\openssl\dtls1.h, which will collide with Poco'c include of winsock2.h. You can solve that temporarily by changing <winsock.h> to <winsock2.h> in dtls1.h until OpenSSL has released a fix (at least that's my interpretation of what you can read on the internet about OpenSSL and their recently added include of winsock.h instead of winsock2.h).